This invention relates to chromium nitride coated filament wear guides and to the method of their manufacture.
As is well-known to workers in the field, in the handling of threads, yarns and the like, the guide structures over which the threads, etc. pass must be characterized by both excellent wear resistance and controlled friction as between the running thread and the guide surface. The problem has been somewhat magnified in recent years by the development and growing use of synthetic threads which are even more abrasive than naturally occurring fibers. For efficient production without undue down time to replace the guide members (along with the replacement cost thereof) it is highly desirable that such guides serve for very extended periods without causing filament damage. To provide a method of readily and easily producing such desirable new guides is one of the principal purposes of this invention. As is set out below in some detail the nitrided structures produced as herein taught are very well suited for textile wear guide purposes.
We recognize that, generally speaking, the use of a chromic oxide surface on a thread guide device is old in that art. See U.S. Pat. No. 3,080,135 of R. P. Steijn. Such patent discloses the making of textile wear guides having a chromic oxide operating surface by the following methods:
A. from solid chromic oxide pieces -- ceramics -- followed by grinding to the desired smoothness; and PA1 B. flame spraying of chromic oxide onto a substrate, then followed by grinding, etc., to the desired smoothness.
In practice we find that such prior art devices suffer various shortcomings which are readily overcome by the use of the present invention. The solid ceramics not only are relatively brittle, with the attendant handling and installation problems in commercial use, but also are generally made with a high friction surface which necessitates complex secondary finishing operations to provide the controlled frictional surface. Similarly, the ceramic coated metals, produced by flame spraying do not inherently have useable low friction surfaces as fabricated. Both of these materials are rather rough, have undesirable snagging characteristics, and cause yarn damage unless extensively finished in a secondary operation.
It also should be noted that many textile wear guides are very small structures. As is alluded to in the Steijn patent, the chromic oxide flame spraying technique is mostly limited in application to parts having relatively simple geometry because of the line-of-sight nature of the process.
In our copending patent application entitled "Wear Resistant Filament Wear Guides and Method of Making the Same" Ser. No. 388,812 filed Aug. 16, 1973, now abandoned and replaced by continuation-in-part application Ser. No. 606,755 filed Aug. 22, 1975, we disclose a novel method of making improved chromium oxide surfaced filament wear devices. We have now discovered, very surprisingly, that a chromium nitride surface formed upon a non-particulate continuous, integral chromium surface has outstanding resistance to abrasive filament wear. Such surface is solely chromium nitride.
The nitriding of chromium to improve its hardness and wear resistance has been previously disclosed--see German Offenlegungsschrift No. 1,902,209, Jan. 17, 1969 of A. U. Seybolt. Although it is known that chromium can be hardened with nitrogen no one has recognized the remarkable resistance to abrasive filament wear that this material possesses. Although chromium nitride is hard it is considerably softer than materials such as chromium boride or titanium carbide. Nevertheless, the abrasive filament wear resistance of chromium nitride is far superior to that of either chromium boride or titanium carbide as will be shown later in this specification. We find that chromium nitride as formed by our process is comparable to or only slightly inferior to the chromium oxide of our copending patent application described above. Both of these materials are the most resistant to abrasive filament wear of all the materials that we have investigated. There are numerous other advantages to chromium nitride as will be subsequently described.
We also note U.S. Pat. No. 3,743,551 directed to razor blades having exceptionally thin coatings of chromium oxide and nitride thereon. Such coatings, which have a maximum thickness of a few hundred angstrom units do not have the outstanding abrasive filament wear resistance as produced in the devices of our invention.
Another prior art patent is that of Rudness, U.S. Pat. No. 3,787,229 "Low Friction, Wear-Resistant Material". Here spheres or spheroids of various ceramic materials are partially embedded in a matrix or binder layer. Mentioned as binders are various resins, rubber, ceramic, glass and metal which are capable of adhering to the substrate and of retaining the embedded rounded particles thereon. Such patent shows that the resulting surface consists of at least two substances--the hard, discrete particles embedded in a softer matrix binder.
In the present invention, on the other hand, a chromium layer deposited on the filament wear guide shaped substrate is nitrided by reaction with a nitriding reactant to form a surface which is in conformity with the layer. If such layer is smooth and uninterrupted the chromium nitride replicates on this. If the chromium layer has a so-called "matte" finish, i.e., somewhat wavy, so also will the chromium nitride layer reacted thereon. And if the chromium has a crack mosaic, as is oftentimes the case, the chromium nitride conforms to this with the added proviso that the nitride layer or zone is quite continuous.
Thus, in distinction to all of prior art teachings, we have developed a method of forming chromium nitride coated thread guides, or the like, which are not brittle, have excellent handling characteristics, are formed directly upon nitriding without the necessity of a secondary surfacing operation, and are useful in making thread guides of practically any size.
In the present invention, a chromium deposit on a substrate member in the form of a textile wear guide is nitrided.
Accordingly, a principal object of our invention is to provide devices characterized by excellent filament wear abrasion resistance and low friction to filaments passing thereover by the practice of the present process.
Another object of our invention is to provide a novel method of producing filament wear guides having a nitrided chromium surface zone thereon.